In hearing stacks, angular contact ball bearings are arranged in a manner that provides increased capacity with successive rows of ball bearings. In high capacity applications, such as down-hole drilling motors, it is not uncommon to have eight or more rows to accommodate the required loads. Each additional row of ball bearings requires the addition of a “shaft bearing race” and a “housing bearing race”. To operate properly and prevent damage to the bearing stack, the shaft bearing races that rotate with the shaft must be “clamped” axially to prevent any one or more shah bearing races rotating independently of the others in the shaft bearing race stack. Additionally, the housing bearing races must be clamped to remain stationary with the housing to ensure all races in the housing bearing race stack rotate together and prevent any housing bearing races from rotating independently of the others. If the shaft or housing bearing races are not clamped to prevent relative rotation with respect to an adjacent bearing race, the surfaces between those races can wear and the stack does not perform as desired. Worn surfaces between adjacent bearing races will reduce the axial clamping force on the stacked races. This in turn causes additional individual rotation and increased wear between adjacent clamped surfaces. The result is uneven load sharing between rows of ball bearings and ultimately premature ball bearing and race failure.
Referring to FIGS. 1, 1a and 1b, with respect to shaft 100 and housing 102, the common method used to achieve an adequate clamping three for the inner races 104 and outer races 106 of the bearing stack 108 involves accurate measurement, assembly, disassembly and final assembly of shaft 100 and housing 102 components. The depicted bearing assembly 110 is an oil sealed unit. Mud lubricated bearing assemblies are another type that uses a similar approach to clamping shaft and housing bearing races. When the shaft connection is assembled and torqued, a shim 112 of a particular length must be selected to apply an appropriate clamping force to shaft bearing races 104 when the stack is assembled. The desired axial clamping force must be adequate to ensure that all shah bearing races 104 remain stationary with shaft 100, but not of a magnitude that will plastically deform, or crush, the Shaft bearing races 104. A similar shim 114 is used to apply an appropriate clamping force to housing bearing races 106. In some assemblies, the tolerance of the shim width can be as small as 0.005 inches to operate properly.
The applied axial threes to the two sets of bearing races, i.e. the shaft and housing bearing races, should provide the same axial deflection in compression in both the shaft bearing races and the housing bearing races. This will ensure favourable load sharing between the rows of ball bearings and maintain the proper axial movement between on-bottom and off-bottom loading.
Referring to FIGS. 2, 2a and 2b, a common method used for shaft shim 112 selection is depicted. The drilling motor is assembled to the stage as shown in FIG. 2. The space 116 between the bearing stack 108 and shim shoulder 118 of the shaft connection is measured and an appropriate length for shaft shim 112 is selected to provide the desired clamping force to the shaft bearing races 104. The shaft connection is then disassembled, the selected shim 112 installed, and the connection re-installed and torqued tight.
The method used for the selection of housing shim 114 is described with reference to FIGS. 3, 3a, 4a and 4b. The drilling motor is assembled to the stage as shown in FIGS. 3 and 3a. An oversized shim 120 of a known length is utilized to provide a gap 122 at the shoulder of the housing connection, FIG. 3a. The gap 122 is measured, the bearing housing 102 disassembled, and the appropriate length housing shim 114 is installed, FIGS. 4 and 4a. The bearing housing 102 is then reinstalled and the housing connection torqued tight.
Although the procedure appears straight forward, many situations arise that tend to lengthen the assembly procedure. For instance, when the shims 112 and 114 are installed and the connections are torqued tight, the “end play” for on-bottom and off-bottom loading is checked to ensure the shaft bearing races 104 and the housing bearing races 106 have been clamped appropriately. If not, the assembly and shim process must be repeated. If the correct shim is not available for either the shaft or housing, then shims must be modified or machined to the correct width to complete the assembly. It is not uncommon to take several hours to complete this task if mistakes are made in the process. The installing and removing the assembled parts can be difficult and time consuming.